Currently each tag only has one question (and neither tag has a wiki summary).
For health the question was Would wearing heavy clothing be enough to solve the health effects of reduced gravity on other planets?.
For medical the question was What medical conditions can impede someone from going into space?.
health would presumably be concerned with behavior, diet, and environment, typically over longer time spans. (Also would psychological health fit under health? There is one psychology question.)
medical would seem to be more concerned with medical treatment and (as in the sole existing question) interactions of medical conditions with space travel. (Medical research questions may or may not be on-topic.)
Someone with a clue should write wiki summaries that would help distinguish these two related tags. (I am assuming that they are not so closely related as to be synonyms.)